For example, in many cases, optical fibers are connected to each other by removing coverings of the optical fibers and fusion-splicing bare fibers of the optical fibers to each other. Such portions of the optical fibers where the coverings have been removed are vulnerable to external forces and may be broken when any impact or vibration is exerted to the optical fibers. In order to protect a fusion splice portion from external forces, therefore, the fusion splice portion of the optical fibers needs to be reinforced. Meanwhile, with regard to an optical fiber for transmitting pump light that is used in a fiber laser or the like, the pump light is likely to leak from a fusion splice portion. Thus, it is necessary to prevent pump light from leaking from such a fusion splice portion.
Accordingly, there has been proposed a structure that holds a fusion splice portion within a metal housing in a state in which the fusion splice portion is recoated with a resin having a refractive index equal to or lower than a refractive index of a cladding or a covering of an optical fiber while the resin is covered with a reinforcing sleeve (see, e.g., Patent Literature 1). With such a structure, the recoated resin can prevent pump light from leaking out of the fusion splice portion, and the reinforcing sleeve and the metal housing can protect the fusion splice portion from external forces.
Meanwhile, when a laser apparatus including such a fusion splice portion is to be manufactured, it is necessary to ascertain whether no defects are found at a fusion splice portion of optical fibers. Such defects of optical fibers at a fusion splice portion are found by operating a laser apparatus under the same conditions as those in a normal operation or under different conditions than those in a normal operation and measuring the temperature of the fusion splice portion to determine whether unusual heat generation is caused to the fusion splice portion. Such temperature measurement is often performed by detecting an infrared ray radiated from the fusion splice portion with an infrared analysis apparatus (thermography apparatus).
However, with the structure disclosed in Patent Literature 1, since the fusion splice portion is covered with a resin and a reinforcing sleeve, infrared rays radiated from the fusion splice portion are blocked by the resin and the reinforcing sleeve. Therefore, it is difficult to accurately measure infrared rays radiated from the fusion splice portion with an infrared analysis apparatus. Even if unusual heat generation is caused in the fusion splice portion, the infrared analysis apparatus cannot detect an increase of the temperature of the fusion splice portion. Accordingly, any defects may be unable to be detected at the fusion splice portion within a range of the rated output. In some cases, unusual heat generation may be found only when the optical fibers are burnt out.